Polycarbonate is recognized as an engineering thermoplastic having excellent toughness, clarity, ignition resistance, dimensional stability, good insulation properties and corona resistance, and high impact and creep resistance over a wide temperature range. On account of its excellent overall combination of properties, polycarbonates are utilized in a wide variety of applications such as appliance and power tool housings, automotive headlight and taillight lenses, aircraft parts, airport runway markers, motorcycle helmets and face guards, windshields, and the like. However, with the ever expanding demand for greater impact-efficient products, resin producers have sought to achieve enhanced properties by blending or otherwise incorporating acrylic polymers into polycarbonate resins. In addition to improved impact resistance, incorporation or blending of acrylic polymers, particularly polymethyl methacrylate, with polycarbonate may also be expected to enhance scratch resistance and UV resistance.
U.S. Pat. No. 4,469,852 to Tyrell et al. discloses a composition based on an aromatic polycarbonate in admixture with impact modifying quantities of a modified aromatic polycarbonate to which are grafted polymers of a long chain alkenyl compound. The modified aromatic polycarbonate is modified to include alkenyl substituted aromatic constituents. Alkyl (alkyl)acrylates wherein the ester alkyl portion contains from 3 to 20 carbon atoms are grafted to the alkenyl groups of the modified aromatic polycarbonate at the olefinic unsaturation site thereof. The process of Tyrell et al., therefore, has the inherent disadvantage of requiring the copolymerization of a polycarbonate copolymer formed from specially modified comonomers containing alkenyl substituted aromatic constituents. Accordingly, the process of Tyrell et al. involves the synthesis or procurement of compounds which are not readily available as commodity chemicals. An additional disadvantage is that any unreacted carbon-carbon double bonds in the alkenyl group will lead to polymeric compositions which are sensitive to oxidative and thermal degradation. Moreover, Tyrell et al. teach a free radical graft polymerization process which is conducted entirely in the liquid phase and which involves the use of a solvent such as cyclohexane from which the graft copolymer must normally be separated. Also, because of the higher mass transfer resistances encountered in the liquid phase, the overall or apparent reaction rate for the graft copolymerization process is slower than would be expected from a process wherein the polycarbonate to which the alkyl (alkyl)acrylates are to be grafted is in the solid phase.
Accordingly, a process for producing polycarbonate-graft-poly(alkyl (alkyl)acrylate) copolymers from conventional polycarbonate polymers and/or precursors free of alkenyl groups, and which is conducted by contacting the alkyl (alkyl)acrylates with the polycarbonate being in the solid phase would overcome many of the problems of the prior art. In particular, the absence of carbon-carbon double bonds from the polycarbonate used to produce the polycarbonate-graft-poly(alkyl (alkyl)acrylate) copolymer would tend to provide graft copolymers having greater resistance to thermal and oxidative degradation, and would eliminate the need for synthesizing specially modified polycarbonates. Also, a graft copolymerization which is conducted with the polycarbonate base polymer in the solid phase would eliminate the need for solvents and for separation processes for removing the product copolymer from the solvents, and would result in lower mass transfer resistance and higher overall reaction rates.